"Do you/the west then accept the result as legitimate, or do you try and find another reason to reject the referendum?"
1. We don't need another reason, that vote ISN'T legitimate...95%. A charade and a poor one at that, if they wanted it to be believable, they could have at least made it a closer illegitimate election.
2. Crimea seceding is reason enough for me, especially when it's seceding to Russia and not only breaking up a country, but strengthening a power the West has no reason to see strengthened...hence why this whole mess has come so far, if the West was OK with closer Ukrainian-Russian ties and Russia OK with closer Ukrainian-Western ties, we'd all be happy and home by now...but that's not the case. It's a zero-sum game, someone's going to win and someone's going to lose, and Vladimir Putin thinks he can score another win over the West right now and increase Russian might and influence both in his own massive sphere as well as on the world stage and, for the moment--that's a damn good bet from where he's sitting. What's the West going to do? He clearly doesn't care about sanctions, and between Kosovo, Iraq and staunchly anti-war populations, unless atrocities were committed, we'd likely have no chance of intervention (and even then there's a chance we'd let it go...look at Syria...I *STILL* say we should have intervened there, if not out of principle than out of political interest and to keep Putin from expanding influence there as well.)
Either Crimea will join Russia amid scandal and sanctions and the West will be the worse for it, or there will be a Civil War and we may perhaps arm Ukraine (as they're asking) but even then...with the Neo-Nazi element (minority force though it is, it's still present and still as big an issue as the Al-Qaeda elements were when we were thinking of arming Syrian rebels) even that's in question.